Cascaded arrangement of two-mode Bragg gratings in multiplexing applications

ABSTRACT

Aspects described herein include an optical apparatus comprising an input port configured to receive an optical signal comprising a plurality of wavelengths, and a plurality of output ports. Each output port is configured to output a respective wavelength of the plurality of wavelengths. The optical apparatus further comprises a first plurality of two-mode Bragg gratings in a cascaded arrangement. Each grating of the first plurality of two-mode Bragg gratings is configured to reflect a respective wavelength of the plurality of wavelengths toward a respective output port of the plurality of output ports, and transmit any remaining wavelengths of the plurality of wavelengths.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 16/814,721, filed Mar. 10, 2020. The aforementioned relatedpatent application is herein incorporated by reference in its entirety.

TECHNICAL FIELD

Embodiments presented in this disclosure generally relate to opticalmultiplexing, and more specifically, to wavelength division multiplexing(WDM) using a cascaded arrangement of two-mode Bragg gratings

BACKGROUND

WDM schemes support multiple channels through a light-carrying medium,such as an optical waveguide or an optical fiber. WDM schemes aretypically distinguished by the spacing between wavelengths. For example,a “normal” WDM system supports 2 channels spaced apart by 240 nanometers(nm), a coarse WDM (CWDM) system supports up to eighteen (18) channelsthat are spaced apart by 20 nm, and a dense WDM (DWDM) system supportsup to eighty (80) channels that are spaced apart by 0.4 nm. Due to thewavelength spacing, a CWDM system tends to be more tolerant than a DWDMsystem and does not require high-precision controlled laser sources. Asa result, a CWDM system tends to be less expensive and consumes lesspower.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above-recited features of the presentdisclosure can be understood in detail, a more particular description ofthe disclosure, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate typicalembodiments and are therefore not to be considered limiting; otherequally effective embodiments are contemplated.

FIG. 1 is a diagram of an exemplary optical apparatus, according to oneor more embodiments.

FIGS. 2 and 3 are diagrams of exemplary silicon-on-insulator (SOI) basedoptical waveguides, according to one or more embodiments.

FIG. 4 is a diagram illustrating exemplary implementations of two-modeBragg gratings with different sidewall corrugation shapes, according toone or more embodiments.

FIGS. 5 and 6 are diagrams of exemplary implementations of ademultiplexer with a cascaded arrangement of two-mode Bragg gratings,according to one or more embodiments.

FIGS. 7 and 8 are diagrams of exemplary implementations of ademultiplexer with mitigated crosstalk, according to one or moreembodiments.

FIG. 9 are graphs illustrating operation of the two-mode Bragg gratingsas bandpass filters, according to one or more embodiments.

FIG. 10 are graphs illustrating operation of the two-mode Bragg gratingsas low-pass filters, according to one or more embodiments.

FIG. 11 is a graph illustrating operation of the two-mode Bragg gratingsas bandpass filters having partially overlapping passbands, according toone or more embodiments.

FIG. 12 illustrates a method of demultiplexing using a cascadedarrangement of two-mode Bragg gratings, according to one or moreembodiments.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneembodiment may be beneficially used in other embodiments withoutspecific recitation.

DESCRIPTION OF EXAMPLE EMBODIMENTS Overview

In one embodiment, an optical apparatus comprising an input portconfigured to receive an optical signal comprising a plurality ofwavelengths, and a plurality of output ports. Each output port isconfigured to output a respective wavelength of the plurality ofwavelengths. The optical apparatus further comprises a first pluralityof two-mode Bragg gratings in a cascaded arrangement. Each grating ofthe first plurality of two-mode Bragg gratings is configured to reflecta respective wavelength of the plurality of wavelengths toward arespective output port of the plurality of output ports, and transmitany remaining wavelengths of the plurality of wavelengths.

In another embodiment, an optical apparatus comprises a plurality ofreceivers and a demultiplexer comprising an input port configured toreceive an optical signal comprising a plurality of wavelengths, and aplurality of output ports. Each output port is configured to output arespective wavelength of the plurality of wavelengths to a respectivereceiver of the plurality of receivers. The demultiplexer furthercomprises a first plurality of two-mode Bragg gratings in a cascadedarrangement. Each grating of the first plurality of two-mode Bragggratings is configured to reflect a respective wavelength of theplurality of wavelengths toward a respective output port of theplurality of output ports, and transmit any remaining wavelengths of theplurality of wavelengths.

EXAMPLE EMBODIMENTS

To achieve a WDM-based optical transceiver module with a small size,optical multiplexing and demultiplexing (mux/demux) functionality may beimplemented in (or integrated with) a photonic integrated circuit (IC)of the optical transceiver module. Low optical losses with the opticalmux/demux are preferable to support a lower-power optical communicationsystem. Further, optical mux/demux having flat-top passbands arebeneficial to eliminate the temperature control of the laser and willreduce the total power consumption of the optical communication system.

According to embodiments described herein, an optical apparatuscomprises an input port configured to receive an optical signalcomprising a plurality of wavelengths, and a plurality of output ports,wherein each output port is configured to output a respective wavelengthof the plurality of wavelengths. The optical apparatus further comprisesa plurality of two-mode Bragg gratings in a cascaded arrangement. Eachgrating is configured to reflect a respective wavelength of theplurality of wavelengths toward a respective output port, and transmitany remaining wavelengths of the plurality of wavelengths. In someembodiments, the two-mode Bragg gratings are formed in opticalwaveguides of a silicon photonic chip. The two-mode Bragg gratings mayhave sidewall corrugation shapes, such as a rectangle shape, a sineshape, or a cosine shape.

Beneficially, using the cascaded arrangement of two-mode Bragg gratingsprovides the multiplexer and/or the demultiplexer with a relativelyflat-top passband, and silicon nitride or silicon oxynitride-basedtwo-mode Bragg gratings can be used to eliminate the temperature controlon the laser source and/or to reduce the power consumption of theoptical apparatus. Further, the two-mode Bragg gratings may be capableof achieving very low insertion loss, such that the multiplexer and/orthe demultiplexer has a low insertion loss, e.g., less than 1-2 decibels(dB). Further, the two-mode Bragg gratings may have much wider passbandsand greater fabrication tolerances.

FIG. 1 is a diagram 100 of an exemplary optical apparatus, according toone or more embodiments. In some embodiments, the optical apparatusrepresents an optical transceiver module integrated into a siliconphotonic chip. Other implementations of the optical apparatus are alsocontemplated.

The optical apparatus comprises a plurality of transmitters 105-1,105-2, 105-3, . . . , 105-M (generically, a transmitter 105) thatprovide optical signals via a respective plurality of optical links110-1, 110-2, 110-3, . . . , 110-M (generically, an optical link 110) toa multiplexer 115. In some embodiments, each transmitter 105 comprises alaser source generating a respective optical signal (e.g., anunmodulated continuous wave (CW) optical signal) having a respectivewavelength. The wavelengths of the optical signals may be selectedaccording to a predefined multiplexing scheme, such as WDM, DWDM, orCWDM. Each transmitter 105 may further comprise an optical modulatorconfigured to modulate the respective optical signal, and may furthercomprise circuitry for further processing of the respective opticalsignal. In some embodiments, the optical links 110 are opticalwaveguides formed in a silicon photonic chip. In other embodiments, theoptical links 110 are optical fibers.

The multiplexer 115 combines the several optical signals into amultiplexed optical signal that is output onto an optical link 120. Insome embodiments, the multiplexer 115 comprises a CWDM multiplexer,although implementations using other WDM schemes are also contemplated.In some embodiments, the optical link 120 is an optical waveguide formedin the silicon photonic chip. In other embodiments, the optical link 120is an optical fiber.

A demultiplexer 125 is communicatively coupled with the multiplexer 115via the optical link 120. The demultiplexer 125 demultiplexes themultiplexed optical signal transmitted by the optical link 120 into aplurality of optical signals. In some embodiments, the demultiplexer 125comprises a CWDM demultiplexer, although other implementations are alsocontemplated. The plurality of optical signals is provided from thedemultiplexer 125 via a respective plurality of optical links 130-1,130-2, 130-3, . . . , 130-N (generically, an optical link 130) to aplurality of receivers 135-1, 135-2, 135-3, . . . , 135-N (generically,a receiver 135). In some embodiments, the optical links 130 are opticalwaveguides formed in the silicon photonic chip. In other embodiments,the optical links 130 are optical fibers. In some embodiments, eachreceiver 135 comprises an optical demodulator to demodulate therespective optical signal, and may further comprise circuitry forfurther processing of the respective optical signal.

In some embodiments, and as will be discussed in greater detail, themultiplexer 115 and/or the demultiplexer 125 comprises two-mode Bragggratings in a cascaded arrangement. Beneficially, using the cascadedarrangement of two-mode Bragg gratings provides the multiplexer 115and/or the demultiplexer 125 with a relatively flat-top passband, andmay be used to eliminate the temperature control on the laser source ofthe transmitters 105 and/or to reduce the power consumption of theoptical apparatus. Further, the two-mode Bragg gratings may be capableof achieving very low insertion loss, such that the multiplexer 115and/or the demultiplexer 125 has an insertion loss of less than 1-2 dB.

FIGS. 2 and 3 are diagrams 200, 300 of exemplary silicon-on-insulator(SOI) based optical waveguides, according to one or more embodiments.The features of the diagrams 200, 300 may be used in conjunction withother embodiments. For example, the multiplexer 115 and/or demultiplexer125 of FIG. 1 may be implemented in a silicon photonic chip using theSOI structures illustrated in the diagrams 200, 300.

In some embodiments, a silicon substrate 205 comprises a bulk silicon(Si) substrate in which one or more features or materials for activeoptical device(s) to be produced (e.g., a laser, detector, modulator,absorber) are pre-processed. The thickness of the silicon substrate 205may vary depending on the specific application. For example, the siliconsubstrate 205 may be the thickness of a typical semiconductor wafer(e.g., 100-700 microns), or may be thinned and mounted on anothersubstrate.

The diagrams 200, 300 each depict the silicon substrate 205, aninsulator layer 210 disposed above the silicon substrate 205, and anoptical waveguide 215 formed in a waveguide layer 220 disposed above theinsulator layer 210. In some embodiments, the insulator layer 210comprises a buried oxide (BOX) layer formed of silicon dioxide. Thethickness of the insulator layer 210 may vary depending on the desiredapplication. In some embodiments, the thickness of the insulator layer210 may range from less than one micron to tens of microns. In someembodiments, the waveguide layer 220 is formed of elemental Si (e.g.,monocrystalline or polycrystalline Si). In other embodiments, thewaveguide layer 220 may be formed of other suitable semiconductormaterials, such as silicon nitride or silicon oxynitride deposited onthe insulator layer 210. The thickness of the waveguide layer 220 mayrange from less than 100 nm to greater than a micron. More specifically,the waveguide layer 220 may be between 100-300 nm thick.

In the diagram 300, the optical waveguide 215 is formed as a ridgewaveguide comprising a ridge 310 projecting from a base 305. The ridgewaveguide generally confines a propagating optical signal within aportion of the waveguide layer 220. In some embodiments, the waveguidelayer 220 has a thickness between 3-5 microns. In some embodiments, thewidth of the ridge 310 (as shown, in the left-right direction) isbetween 3-5 microns. With such dimensioning, the diameter of the opticalmode may be 4-5 microns.

As mentioned above, grating patterns may be etched along the sidewallsof the optical waveguide 215 to form the two-mode Bragg gratings of themultiplexer 115 and/or demultiplexer 125. FIG. 4 is a diagram 400illustrating exemplary implementations of a two-mode Bragg grating withdifferent sidewall corrugation shapes, according to one or moreembodiments. The features of the diagram 400 may be used in conjunctionwith other embodiments. For example, the sidewall gratings may be usedby the two-mode Bragg grating to transmit one wavelength and reflectanother wavelength of light propagating through the optical waveguide215.

The diagram 400 depicts a mode multiplexer 405 and a two-mode Bragggrating 410. A first arm 415 of the mode multiplexer 405 propagates afundamental mode 425 (e.g., a fundamental transverse electric (TE)mode), which is propagated to the two-mode Bragg grating 410. Thetwo-mode Bragg grating 410 comprises sidewalls 435-1, 435-2 that have agrating pattern with a corrugation period Λ₁ and a depth d₁. Althoughthe corrugation shapes of the sidewalls 435-1, 435-2 are shown as beinga rectangle shape, alternate shapes such as a sine shape (as in gratingpattern 440), a cosine shape, etc. are also contemplated.

The grating pattern may be formed, e.g., by deep etching into an edge ofan optical waveguide to create the periodic grating pattern along thelength of the optical waveguide. As shown, the two-mode Bragg grating410 transmits a first mode (e.g., the fundamental mode 425) and reflectsa second mode (e.g., a second-order mode 430) to the mode multiplexer405. A second arm 420 of the mode multiplexer 405 propagates thesecond-order mode. Other implementations of mode multiplexers 405 arealso contemplated, such as asymmetric Y-junction mode multiplexers.

In some embodiments, the grating patterns are formed with a siliconnitride or silicon oxynitride material. For example, the silicon nitrideor silicon oxynitride material may be deposited above a silicon oxidelayer and the optical waveguide is formed using a dry etching process.Both silicon nitride and silicon oxynitride have thermo-opticcoefficients smaller than that of elemental silicon, which results inthe two-mode Bragg gratings (and the associated optical apparatus) beingless sensitive to temperature variations during operation. In somecases, the lower temperature sensitivity means that no thermal tuning ofthe optical apparatus is required during operation.

The grating patterns may have any suitable alternate implementation. Forexample, one or more grating patterns may be formed using a buriedgrating layer. Further, in cases where the length of a two-mode Bragggrating is sufficiently long (e.g., implemented within an opticalfiber), the sidewall gratings may be spaced apart from each other (e.g.,at different positions along the length of the first Bragg grating).

FIGS. 5 and 6 are diagrams 500, 600 of exemplary implementations of ademultiplexer 505, 605 with a cascaded arrangement of two-mode Bragggratings, according to one or more embodiments. The features illustratedin the diagrams 500, 600 may be used in conjunction with otherembodiments. For example, mode multiplexers and two-mode Bragg gratingsincluded in the demultiplexers 505, 605 may be configured as shown inFIG. 4.

In the diagram 500, the demultiplexer 505 comprises an input port 510and a plurality of two-mode Bragg gratings 515-0, 515-1, 515-2 (whichare also referred to herein as “gratings” or “Bragg gratings”) in acascaded arrangement (which may alternately be referred to as a “serial”arrangement). Each grating 515-0, 515-1, 515-2 reflects a respectivewavelength, and transmits any remaining wavelengths. For example, thegrating 515-0 reflects a first wavelength via a drop port, and transmitsat least one wavelength via an output port to gratings 515-1, 515-2 thatare downstream of the grating 515-0.

The gratings 515-0, 515-1, 515-2 may have any suitable filter responsesfor separating the respective wavelength for reflecting. In someembodiments, the gratings 510-0, 515-1, 515-2 are bandpass filters,which may have non-overlapping or partially overlapping passbands. Forexamples, the gratings 515-0, 515-1, 515-2 may have partiallyoverlapping passbands with a center wavelength and an upper roll-offwavelength selected such that a range of the respective wavelengthreflected by the grating 510-0, 515-1, 515-2 is entirely includedbetween the center wavelength and the upper roll-off wavelength. Inother embodiments, the gratings 515-0, 515-1, 515-2 are low-pass filtersand may have successively greater roll-off wavelengths.

In some embodiments, the demultiplexer 505 further comprises a pluralityof mode multiplexers 520-0, 520-1, 520-2. Each mode multiplexer of theplurality of mode multiplexers 520-0, 520-1, 520-2 receives thewavelength reflected by a respective grating 515-0, 515-1, 515-2. Eachmode multiplexer converts the mode of the reflected wavelength (e.g., afirst-order TE mode) into a fundamental TE mode. The plurality of modemultiplexers 520-0, 520-1, 520-2 may have any suitable implementation,e.g., using on-resonance and off-resonance switching rings. Each modemultiplexer 520-0, 520-1, 520-2 has an output that is coupled with arespective output port 525-0, 525-1, 525-2 of a plurality of outputports 525-0, 525-1, 525-2, 525-3 of the demultiplexer 505. In otherembodiments, the plurality of mode multiplexers 520-0, 520-1, 520-2 maybe omitted, such that the gratings 515-0, 515-1, 515-2 provide thereflected wavelengths (e.g., as a first order or higher mode) directlyto the output ports 525-0, 525-1, 525-2.

Thus, responsive to receiving an optical signal 530 comprising aplurality of wavelengths λ₀, λ₁, λ₂, λ₃ at the input port 510, thegrating 515-0 reflects the wavelength λ₀ and transmits the remainingwavelengths λ₁, λ₂, λ₃. The mode multiplexer 520-0 receives thewavelength λ₀ and provides the wavelength λ₀ (with the mode converted toa fundamental mode) to the output port 525-0 as an optical signal 535-0.The grating 515-1 receives the wavelengths λ₁, λ₂, λ₃, reflects thewavelength λ₁, and transmits the remaining wavelengths λ₂, λ₃. The modemultiplexer 520-1 receives the wavelength λ₁ and provides the wavelengthλ₁ (with the mode converted to a fundamental mode) to the output port525-1 as an optical signal 535-1.

The grating 515-2 receives the wavelengths λ₂, λ₃, reflects thewavelength λ₂, and transmits the remaining wavelength λ₃. The modemultiplexer 520-2 receives the wavelength λ₂ and provides the wavelengthλ₂ (with the mode converted to a fundamental mode) to the output port525-2 as an optical signal 535-2. The remaining wavelength λ₃ isprovided from the grating 515-2 to the output port 525-3 as an opticalsignal 535-3.

In the demultiplexer 505, the grating 515-2 represents a “last” gratingin the cascaded arrangement of the gratings 515-0, 515-1, 515-2. Here,the grating 515-2 reflects a “second-to-last” wavelength (i.e., thewavelength λ₂) of the plurality of wavelengths λ₀, λ₁, λ₂, λ₃ toward theoutput port 525-2, and transmits a “last” wavelength (i.e., thewavelength λ₃) to the output port 525-3.

In the diagram 600, the demultiplexer 605 comprises the input port 510,the plurality of output ports 525-0, 525-1, 525-2, 525-3, and a cascadedarrangement of the gratings 515-0, 515-1, 515-2 and a grating 515-3. Theoperation of the demultiplexer 605 is generally similar to that of thedemultiplexer 505. However, the grating 515-3 receives the wavelength λ₃from the grating 515-2, and reflects the wavelength λ₃. Thedemultiplexer 605 further comprises a mode multiplexer 520-3 thatreceives the wavelength λ₃ and provides the wavelength λ₃ (with the modeconverted to a fundamental mode) to the output port 525-3 as the opticalsignal 535-3. In another embodiment, the mode multiplexer 520-3 may beomitted.

In some embodiments, the output of the grating 515-3 (e.g., a transmitport) is coupled with an optical absorber 610. In some embodiments, theoptical absorber 610 comprises a heavily-doped silicon waveguide.Beneficially, the optical absorber 610 mitigates reflections of opticalsignals, which can further improve the signal-to-noise ratio (SNR) ofthe optical signal 535-3.

In the demultiplexer 605, the grating 515-3 represents a “last” gratingin the cascaded arrangement of the gratings 515-0, 515-1, 515-2, 515-3.Here, the grating 515-3 reflects a “last” wavelength (i.e., thewavelength λ₃) of the plurality of wavelengths λ₀, λ₁, λ₂, λ₃ toward theoutput port 525-3.

FIGS. 7 and 8 are diagrams 700, 800 of exemplary implementations of ademultiplexer 705, 805 with mitigated crosstalk, according to one ormore embodiments. The features illustrated in the diagrams 700, 800 maybe used in conjunction with other embodiments. For example, modemultiplexers and two-mode Bragg gratings included in the demultiplexers705, 805 may be configured as shown in FIG. 4.

In the diagram 700, the demultiplexer 705 comprises the input port 510,the plurality of output ports 525-0, 525-1, 525-2, 525-3, a cascadedarrangement of the gratings 515-0, 515-1, 515-2, and the plurality ofmode multiplexers 520-0, 520-1, 520-2. The operation of thedemultiplexer 705 is generally similar to that of the demultiplexer 505,discussed above.

The demultiplexer 705 further comprises a second plurality of two-modeBragg gratings 710-0, 710-1, 710-2. Each grating 710-0, 710-1, 710-2receives a wavelength reflected by a respective grating 515-0, 515-1,515-2, and reflects the wavelength toward a respective output port525-0, 525-1, 525-2. The demultiplexer 705 further comprises a pluralityof mode multiplexers 715-0, 715-1, 715-2. Each mode multiplexer of theplurality of mode multiplexers 715-0, 715-1, 715-2 receives thewavelength reflected by a respective grating 710-0, 710-1, 710-2. Eachmode multiplexer 715-0, 715-1, 715-2 has an output that is coupled witha respective output port 525-0, 525-1, 525-2. The demultiplexer 705further comprises a plurality of optical absorbers 720-0, 720-1, 720-2.Each grating 710-0, 710-1, 710-2 has an output coupled with a respectiveoptical absorber 720-0, 720-1, 720-2, each of which may be configuredsimilarly to the optical absorber 610.

In the diagram 800, the demultiplexer 805 comprises the input port 510,the plurality of output ports 525-0, 525-1, 525-2, 525-3, a cascadedarrangement of the gratings 515-0, 515-1, 515-2, 515-3 and the pluralityof mode multiplexers 520-0, 520-1, 520-2, 520-3. The operation of thedemultiplexer 805 is generally similar to that of the demultiplexer 605,discussed above.

The demultiplexer 805 further comprises a second plurality of two-modeBragg gratings 710-0, 710-1, 710-2, 710-3. Each grating 710-0, 710-1,710-2, 710-3 receives a wavelength reflected by a respective grating515-0, 515-1, 515-2, 515-3 and reflects the wavelength toward arespective output port 525-0, 525-1, 525-2, 525-3. The demultiplexer 805further comprises a plurality of mode multiplexers 715-0, 715-1, 715-2,715-3. Each mode multiplexer of the plurality of mode multiplexers715-0, 715-1, 715-2, 715-3 receives the wavelength reflected by arespective grating 710-0, 710-1, 710-2, 710-3. Each mode multiplexer715-0, 715-1, 715-2, 715-3 has an output that is coupled with arespective output port 525-0, 525-1, 525-2, 525-3. The demultiplexer 705further comprises a plurality of optical absorbers 720-0, 720-1, 720-2,720-3. Each grating 710-0, 710-1, 710-2, 710-3 has an output coupledwith a respective optical absorber 720-0, 720-1, 720-2, 720-3.

Although the combination of a mode multiplexer 405 with a two-mode Bragggrating 410, as shown in FIG. 4, is used in the implementations of thedemultiplexer 505, 605, 705, 805 to perform a demultiplexing function,it will be noted that the combination of the mode multiplexer 405 withthe two-mode Bragg grating 410 may be used to perform a multiplexingfunction. As a result, the combination of the mode multiplexer 405 withthe two-mode Bragg grating 410 may be used in implementations of amultiplexer comprising a plurality of two-mode Bragg gratings in acascaded arrangement.

FIG. 9 are graphs 900-0, 900-1, 900-2, 900-3 illustrating operation ofthe two-mode Bragg gratings as bandpass filters, according to one ormore embodiments. The features illustrated in the graphs 900-0, 900-1,900-2, 900-3 may be used in conjunction with other embodiments. Forexample, the cascaded arrangement in any of the demultiplexers 505, 605,705, 805 may have gratings configured as bandpass filters. As discussedabove, the gratings may have non-overlapping or partially overlappingpassbands.

In the graph 900-0, the first grating in the cascaded arrangementreceives an optical signal comprising a plurality of signal components905-0, 905-1, 905-2, 905-3 at a respective plurality of wavelengths λ₀,λ₁, λ₂, λ₃. A filter response 910-0 of the first grating includes afirst passband 915-0, such that the signal component 905-0 (at thewavelength λ₀) is reflected by the first grating. The remainingwavelengths λ₁, λ₂, λ₃ (represented as a group 920-0 of the signalcomponents 905-1, 905-2, 905-3) are transmitted by the first grating toa second grating in the cascaded arrangement.

In the graph 900-1, the second grating receives the signal components905-1, 905-2, 905-3 at the respective wavelengths λ₁, λ₂, λ₃. A filterresponse 910-1 of the second grating includes a second passband 915-1,such that the signal component 905-1 (at the wavelength λ₁) is reflectedby the second grating. The remaining wavelengths λ₂, λ₃ (represented asa group 920-1 of the signal components 905-2, 905-3) are transmitted bythe second grating to a third grating in the cascaded arrangement.

In the graph 900-2, the third grating receives the signal components905-2, 905-3 at the respective wavelengths λ₂, λ₃. A filter response910-2 of the third grating includes a third passband 915-1, such thatthe signal component 905-2 (at the wavelength λ₂) is reflected by thethird grating. The remaining wavelength λ₃ (represented as a group 920-2of the signal component 905-3) is transmitted by the third grating.

The signal component 905-3 (at the wavelength λ₃) is illustrated in thegraph 900-3. In some embodiments, the signal component 905-3 istransmitted by the third grating to an output port. In otherembodiments, the signal components 905-3 is reflected by a fourthgrating toward the output port. Although the graphs 900-0, 900-1, 900-2,900-3 show one sequence of filtering the signal components 905-0, 905-1,905-2, 905-3 using the cascaded arrangement, other embodiments may havealternate sequences of filtering the signal components 905-0, 905-1,905-2, 905-3.

FIG. 10 are graphs 1000-0, 1000-1, 1000-2, 1000-3 illustrating operationof the two-mode Bragg gratings as low-pass filters, according to one ormore embodiments. The features illustrated in the graphs 1000-0, 1000-1,1000-2, 1000-3 may be used in conjunction with other embodiments. Forexample, the cascaded arrangement in any of the demultiplexers 505, 605,705, 805 may have gratings configured as low-pass filters.

In the graph 1000-0, the first grating in the cascaded arrangementreceives the optical signal comprising the plurality of signalcomponents 905-0, 905-1, 905-2, 905-3. A filter response 1005-0 of thefirst grating includes a first passband 1010-0, such that the signalcomponent 905-0 (at the wavelength λ₀) is reflected by the firstgrating. The remaining wavelengths λ₁, λ₂, λ₃ are transmitted by thefirst grating to a second grating in the cascaded arrangement.

In the graph 1000-1, the second grating receives the signal components905-1, 905-2, 905-3. A filter response 1005-1 of the second gratingincludes a second passband 1010-1, such that the signal component 905-1(at the wavelength λ₁) is reflected by the second grating. The remainingwavelengths λ₂, λ₃ are transmitted by the second grating to a thirdgrating in the cascaded arrangement.

In the graph 1000-2, the third grating receives the signal components905-2, 905-3. A filter response 1005-2 of the third grating includes athird passband 1010-1, such that the signal component 905-2 (at thewavelength λ₂) is reflected by the third grating. The remainingwavelength λ₃ is transmitted by the third grating.

The signal component 905-3 (at the wavelength λ₃) is illustrated in thegraph 1000-3. In some embodiments, the signal component 905-3 istransmitted by the third grating to an output port. In otherembodiments, the signal components 905-3 is reflected by a fourthgrating toward the output port.

As shown, the passbands 1010-0, 1010-1, 1010-2, 1010-3 are all partiallyoverlapping with each other. However, other embodiments may includedifferent combinations of passbands, which may include some passbandsthat are non-overlapping. For example, the cascaded arrangement mayinclude a combination of one or more gratings configured as low-passfilters and one or more gratings configured as bandpass filters.Further, gratings configured as high-pass filters are also contemplated,whether used in isolation or in combination with other types of filters.

FIG. 11 is a graph 1100 illustrating operation of the two-mode Bragggratings as bandpass filters having partially overlapping passbands,according to one or more embodiments. The features illustrated in thegraph 1100 may be used in conjunction with other embodiments. Forexample, the cascaded arrangement in any of the demultiplexers 505, 605,705, 805 may have gratings configured as bandpass filters.

The graph 1100 illustrates the filter responses 910-0, 910-1, 910-2,910-3 for the respective gratings. The filter responses 910-0, 910-1,910-2, 910-3 include the partially overlapping passbands 915-0, 915-1,915-2, 915-3. Each passband 915-0, 915-1, 915-2, 915-3 has a respectivecenter wavelength λ_(C0), λ_(C1), λ_(C2), λ_(C3) and a respective upperroll-off wavelength λ_(R0), λ_(R1), λ_(R2), λ_(R3). The centerwavelengths λ_(C0), λ_(C1), λ_(C2), λ_(C3) and the upper roll-offwavelengths λ_(R0), λ_(R1), λ_(R2), λ_(R3) are selected such that ranges1105-0, 1105-1, 1105-2, 1105-3 surrounding the respective wavelengthsλ₀, λ₁, λ₂, λ₃ are entirely included between the center wavelengthsλ_(C0), λ_(C1), λ_(C2), λ_(C3) and the upper roll-off wavelengthsλ_(R0), λ_(R1), λ_(R2), λ_(R3). Stated another way, a first grating isdesigned such that a range 1105-0 surrounding a first wavelength λ₀ isentirely included between the center wavelength λ_(C0) and the upperroll-off wavelength λ_(R0), a second grating is designed such that arange 1105-1 surrounding a second wavelength λ₁ is entirely includedbetween the center wavelength λ_(C1) and the upper roll-off wavelengthλ_(R1), and so forth. By accommodating the ranges 1105-0, 1105-1,1105-2, 1105-3 in this manner, the partially overlapping passbands915-0, 915-1, 915-2, 915-3 may be spaced closer together to have agreater amount of overlap while maintaining suitable selectivity of thegratings (i.e., to reflect one wavelength but not an adjacentwavelength) in the cascaded arrangement.

In one non-limiting example of a CWDM scheme, four (4) lanes are definedsuch that the wavelength λ₀=1271 nm, the wavelength λ₁=1291 nm, thewavelength λ₂=1311 nm, and the wavelength λ₃=1331 nm. Each of the ranges1105-0, 1105-1, 1105-2, 1105-3 is ±6.5 nm of the respective wavelengthλ₀, λ₁, λ₂, λ₃, such that the range 1105-0 is 1264.5 nm to 1277.5 nm(corresponding to a total range of 13 nm), the range 1105-1 is 1284.5 nmto 1297.5 nm, the range 1105-2 is 1304.5 nm to 1317.5 nm, and the range1105-3 is 1324.5 nm to 1337.5 nm.

Assume that the center wavelength λ_(C0)=1264 nm, the center wavelengthλ_(C1)=1284 nm, the center wavelength λ_(C2)=1304 nm, and the centerwavelength λ_(C3)=1324 nm (corresponding to a channel spacing of 20 nm).As each of the gratings has a passband of 32 nm, the upper roll-offwavelength λ_(R0)=1280 nm, the upper roll-off wavelength λ_(R1)=1300 nm,the upper roll-off wavelength λ_(R2)=1320 nm, and the upper roll-offwavelength λ_(R3)=1340 nm.

In this way, the range 1105-0 (1264.5 nm to 1277.5 nm) is entirelyincluded between the center wavelength λ_(C0) (1264 nm) and the upperroll-off wavelength λ_(R0) (1280 nm) for the first grating, the range1105-1 (1284.5 nm to 1297.5 nm) is entirely included between the centerwavelength λ_(C1) (1284 nm) and the upper roll-off wavelength λ_(R1)(1300 nm) for the second grating, and so forth.

Beneficially, by configuring the gratings to provide the passbands915-0, 915-1, 915-2, 915-3 (FIGS. 9, 11) and the passbands 1010-0,1010-1, 1010-2, 1010-3 (FIG. 10) as relatively wide and flat-toppassbands with a steep edge spectrum response, the demultiplexer tendsto have greater tolerance for fabrication variations, material layerthickness variations, and/or temperature variations. Using a siliconnitride or silicon oxynitride material for the gratings furtherincreases the tolerance for these variations.

While the above example is discussed in terms of partially overlappingpassbands 915-0, 915-1, 915-2, 915-3 for bandpass filters, similartechniques may be used to space the passbands of low-pass filters closertogether while maintaining a suitable selectivity. For example, thecut-off wavelengths of the low-pass filters may be selected such that aminimum margin (e.g., 2 nm) exists between the cut-off wavelength for agrating and the range surrounding the particular wavelength of theoptical signal to be reflected by the grating.

Further, while the demultiplexers 505, 605, 705, 805 have been depictedas a 1-to-4 (1:4) demultiplexer having three (3) or four (4) two-modeBragg gratings in a cascaded arrangement, other configurations of thedemultiplexers 505, 605, 705, 805 are also contemplated. For example,the demultiplexer 125 may include a larger or smaller number of Bragggratings in the cascaded arrangement, different filter responses for thegratings, and so forth.

FIG. 12 illustrates a method 1200 of demultiplexing using a cascadedarrangement of two-mode Bragg gratings, according to one or moreembodiments. The method 1200 may be used in conjunction with otherembodiments, e.g., performed using any of the demultiplexers 505, 605,705, 805 described above.

The method 1200 begins at block 1205, where the demultiplexer receives,at an input port, an optical signal comprising a plurality ofwavelengths. At block 1215, a respective wavelength is reflected using atwo-mode Bragg grating of a cascaded arrangement. At block 1225, themode of the reflected wavelength is converted into a fundamental mode.In some embodiments, the conversion is performed using a modemultiplexer arranged at a drop port of the two-mode Bragg grating.

At block 1235, any remaining wavelengths are transmitted using thetwo-mode Bragg grating. The method 1200 returns from block 1235 to block1215 for each of the two-mode Bragg gratings of the cascadedarrangement. In some embodiments, a last wavelength is reflected by alast grating of the cascaded arrangement. In other embodiments, a lastgrating reflects a second-to-last wavelength and transmits the lastwavelength. At block 1245, individual wavelengths are output atrespective output ports. The method 1200 ends following completion ofblock 1245.

In the preceding, reference is made to embodiments presented in thisdisclosure. However, the scope of the present disclosure is not limitedto specific described embodiments. Instead, any combination of thedescribed features and elements, whether related to differentembodiments or not, is contemplated to implement and practicecontemplated embodiments. Furthermore, although embodiments disclosedherein may achieve advantages over other possible solutions or over theprior art, whether or not a particular advantage is achieved by a givenembodiment is not limiting of the scope of the present disclosure. Thus,the preceding aspects, features, embodiments and advantages are merelyillustrative and are not considered elements or limitations of theappended claims except where explicitly recited in a claim(s).

Aspects of the present disclosure are described with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodimentspresented in this disclosure. It will be understood that each block ofthe flowchart illustrations and/or block diagrams, and combinations ofblocks in the flowchart illustrations and/or block diagrams, can beimplemented by computer program instructions. These computer programinstructions may be provided to a processor of a general purposecomputer, special purpose computer, or other programmable dataprocessing apparatus to produce a machine, such that the instructions,which execute via the processor of the computer or other programmabledata processing apparatus, create means for implementing thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality and operation of possible implementations ofsystems, methods and computer program products according to variousembodiments. In this regard, each block in the flowchart or blockdiagrams may represent a module, segment, or portion of code, whichcomprises one or more executable instructions for implementing thespecified logical function(s). It should also be noted that, in somealternative implementations, the functions noted in the block may occurout of the order noted in the figures. For example, two blocks shown insuccession may, in fact, be executed substantially concurrently, or theblocks may sometimes be executed in the reverse order, depending uponthe functionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts, or combinations of special purpose hardware andcomputer instructions.

In view of the foregoing, the scope of the present disclosure isdetermined by the claims that follow.

We claim:
 1. An optical apparatus comprising: an input port configuredto receive an optical signal comprising a plurality of wavelengths; aplurality of output ports each configured to output a respectivewavelength of the plurality of wavelengths; one or more two-mode Bragggratings (TBGs); and one or more mode multiplexers each configured to:transmit, to a respective TBG of the one or more TBGs, a fundamentalmode of a respective first wavelength of the plurality of wavelengths,wherein the respective TBG is configured to (i) transmit the fundamentalmode of the respective first wavelength and (ii) reflect a respectivesecond wavelength of the plurality of wavelengths; receive therespective second wavelength reflected from the respective TBG; converta mode of the respective second wavelength to a fundamental mode; andoutput the fundamental mode of the respective second wavelength.
 2. Theoptical apparatus of claim 1, wherein each of the one or more modemultiplexers is configured to output the fundamental mode of therespective second wavelength to a respective output port of theplurality of output ports.
 3. The optical apparatus of claim 1, whereineach of the one or more TBGs comprises a silicon nitride or a siliconoxynitride material.
 4. The optical apparatus of claim 1, wherein eachof the one or more TBGs is configured to reflect a first-order mode or asecond-order mode of the respective second wavelength.
 5. The opticalapparatus of claim 1, wherein the one or more TBGs comprise a pluralityof TBGs in a cascaded arrangement, and wherein the one or more modemultiplexers comprise a plurality of mode multiplexers.
 6. The opticalapparatus of claim 5, wherein a last TBG of the cascaded arrangement isconfigured to: reflect a second-to-last wavelength of the plurality ofwavelengths toward a first output port of the plurality of output ports;and transmit a last wavelength of the plurality of wavelengths to asecond output port of the plurality of output ports.
 7. The opticalapparatus of claim 5, wherein a last grating of the cascaded arrangementis configured to: reflect a last wavelength of the plurality ofwavelengths to a first output port of the plurality of output ports. 8.The optical apparatus of claim 1, further comprising: one or more otherTBGs each configured to: receive, from a respective mode multiplexer ofthe one or more mode multiplexers, the fundamental mode of therespective second wavelength; and reflect the respective secondwavelength toward a respective output port of the plurality of outputports.
 9. The optical apparatus of claim 8, further comprising: one ormore optical absorbers, wherein each of the one or more other TBGs hasan output coupled with a respective optical absorber of the one or moreoptical absorbers.
 10. The optical apparatus of claim 1, wherein the oneor more TBGs comprise a plurality of TBGs having non-overlappingpassbands.
 11. The optical apparatus of claim 1, wherein the one or moreTBGs comprise a plurality of TBGs having partially overlappingpassbands.
 12. The optical apparatus of claim 11, wherein each passbandof the partially overlapping passbands has a center wavelength and anupper roll-off wavelength that are selected such that a range of therespective second wavelength reflected by the respective TBG of theplurality of TBGs is entirely included between the center wavelength andthe upper roll-off wavelength.
 13. The optical apparatus of claim 1,wherein the one or more TBGs operate as low-pass filters.
 14. An opticalapparatus comprising: a plurality of receivers; and a demultiplexercomprising: an input port configured to receive an optical signalcomprising a plurality of wavelengths; a plurality of output ports eachconfigured to output a respective wavelength of the plurality ofwavelengths to a respective receiver of the plurality of receivers; oneor more two-mode Bragg gratings (TBGs); and one or more modemultiplexers each configured to: transmit, to a respective TBG of theone or more TBGs, a fundamental mode of a respective first wavelength ofthe plurality of wavelengths, wherein the respective TBG is configuredto (i) transmit the fundamental mode of the respective first wavelengthand (ii) reflect a respective second wavelength of the plurality ofwavelengths; receive the respective second wavelength reflected from therespective TBG; convert a mode of the respective second wavelength to afundamental mode; and output the fundamental mode of the respectivesecond wavelength.
 15. The optical apparatus of claim 14, wherein thedemultiplexer operates as a coarse wavelength division multiplexing(CWDM) demultiplexer.
 16. The optical apparatus of claim 14, wherein theplurality of receivers and the demultiplexer are implemented in aphotonic chip.